The invention concerns a method and a system for the recognition of insulation defects in a circuit designed for connection to a low-ohmic current source or sink having an off-load voltage U.sub.n0.
The terms "current source" and "current sink" each designate a single terminal device (which, of course, can have additional terminals for its supply of power) which can deliver or receive current. For example, a battery having a positive and negative terminal is a combination of a current source and a current sink.
"Low-ohmic" designates that the current source or sink can, in the event of a short in the circuit, supply a sufficiently large current to cause disadvantageous results. In contrast thereto, the current of a high-ohmic or current limited current source remains relatively small and cannot have such effects.
An example of such a low-ohmic current source and sink is an automobile battery which can be connected via a cable to electrical utilities. In the event that a short to ground occurs in a utility due to an insulation defect, for example in the starter, connection to the current source (e.g. the positive terminal of the battery) can lead to high short-circuit current, to heating of the connecting cable, and to a fire in a cable bundle.
Another example of a low-ohmic current source and sink is the current supply for triggering the ignition pellet of an airbag system in a motor vehicle. Normally, both terminals of the ignition pellet are separated from the current source. However, as will be described in more detail below, one terminal is connected to the low-ohmic current source (i.e. the positive terminal of the current source) during internal diagnostic checking of the airbag system. In the event that its other terminal is shorted to ground due to an insulation defect, ignition thereof and consequently of the airbag is unintentionally effected.
In particular, a conventional airbag system generally exhibits two power switches for complete decoupling of the ignition pellet under normal operating conditions of which one is in the current path between the low-ohmic current source (e.g. the positive terminal of the current supply) and one terminal of the ignition pellet and the other in that between the other terminal of the ignition pellet and the low-ohmic current sink (i.e. to ground). When both switches are closed, the ignition pellet ignites. In order to verify its functionability, a system of this type performs repeated internal checks. Towards this end all components of the system are checked, including not only the collision sensors, ignition and self-sufficient capacitors but also, in particular, the ignition pellet and the two power switches. In order to check the ignition pellet, one terminal of the ignition pellet is connected to a current limited current source and the other to a current limited current sink with the power switches open. As a result, a limited diagnostic current flows through the ignition pellet within a range not leading to ignition. The voltage drop across same is checked and evaluated. In order to check the power switch one switch is closed at a time so that a diagnostic current flows, in one case, from the low-ohmic current source through the closed switch and the ignition pellet to the current limited current sink and, in the other case, from the current limited current source through the ignition pellet and the closed switch to the low-ohmic current sink. Hereby, the diagnostic current is only limited by the current limited current source or sink on the side of the open switch; that on the side of the closed switch is, in each case, in parallel with the low-ohmic current source or sink and therefore has no effect. The diagnostic check then consists, in each case, of an evaluation of the current flowing through the switch which can be sampled by a sensing device. However, a short-circuit can--as already mentioned above--thereby lead to unintentional ignition of the airbag. In the first case mentioned above, a short to the low-ohmic current sink (i.e. to ground) nullifies the current limitation of the current sink and--to the extent that it is located on the downstream side of the ignition pellet--allows the full ignition current to flow, leading to unintentional ignition. Similar results obtain, although with reduced probability when, in the second case mentioned above, a short-circuit to the low ohmic current source (a so-called local short to the voltage U of the battery) occurs on the upstream side of the ignition pellet.
The conventional method for preventing such unintentional ignition provides for a check of shorts to ground prior to the switch diagnostic check by connecting the ignition pellet to a diagnostic potential which has a sufficiently low level above ground that the diagnostic current cannot lead to ignition in the event of a short to ground. One measures the diagnostic voltage and, in the event that it lies below a prescribed value, concludes that a short-circuit is present and therefore does not carry out the otherwise subsequent diagnostic check of the switch. In this manner it is possible to prevent an unintentional ignition of the airbag in most cases. Nevertheless, there is still an unacceptably large probability of unintentional ignition during switch diagnostic checks.
It has been recognized that these remaining unintentional ignitions are usually due to insulation defects which are not recognized in the above described short-circuit diagnostic check using relatively low diagnostic voltages. This is due to the presence of, for example, cable locations with worn-through insulation, and metallic particles or slivers, especially in the region of the ignition pellet plug, which do not initially effect contact to ground and therefore are not evident during diagnostic checks using low diagnostic voltage. Only when the relatively high static voltage is applied during the subsequent switch diagnostic check is contact established due to the relatively large attractive electrostatic forces. An initial flow of current causes a "welding together" at the contact location and therefore an additional reduction in the bridging resistance so that a short to ground and an unintentional ignition of the airbag occurs.
The technical problem underlying the invention is to overcome these disadvantages.